Nice to see a long range report. We did 5,231 miles earlier this year in our 22' Toy Hauler at around 6,300lbs weight. It's a rolling dog kennel with a residential sized deep freeze, coolers and lots of metal pet fencing. We went from CA to LA, up to KY and back to CA. I kept it in 5th which is OD on my manual trans and averaged 14.68mpg going around 62-65 the entire trip. Like you there was around 300 miles of non towing which works out to 19mpg for me.
I love these long trips. Nothing like filling the fuel tank and seeing America!